28 October 2012

Primary Molt in an Adult Ring-billed

Adult Ring-billed Gulls typically complete their prebasic molts by the end of October. Here's a series of photos documenting primary growth of a known-age RBGU taken at Rainbow Beach in Chicago, Illinois. The photos are presented in three week intervals from late August through late October.This male was banded in the breeding season of 2007 at the Lake Calumet colony.

Just two white primary tips clearly showing past the tertials on 25 August 2012.
Three primary tips showing past the tertials on 15 September 2012. P8 grew significantly in the last three weeks.
Now 4 primary tips showing beyond the tertials on 06 October 2012.
Finally, the tip to P10 (black) has emerged from underneath P9. 26 October 2012. This generally concludes the prebasic molt as most of the body and flight feathers are renewed by the time P10 is fully grown.
 
The primaries, secondaries and rectrices are all new, crisp, feathers that will get this bird through the winter, spring and most of the next breeding season.

One thing worth noting here is that the mirrors on P10, and particularly P9, are relatively small. This may be an indication as to what an adult this young (6th cycle) should look like, but it's not uncommon for some adults to be without a mirror on P9 entirely. Are these young adult females? Also, might the choppy black markings up the inner web of P8 become "cleaner" as this bird ages? The black marks on the outermost greater primary coverts above P10 are not a sign of "subadultness" - this is an expected feature on adult Ring-billeds.

Rainbow Beach. Chicago, IL. 26 Oct 2012.
To read more about why this bird was tagged, click here.

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